1. Field of the Invention
This invention is related to nuclear magnetic resonance (NMR) techniques and, more specifically, to shimming the magnet of an NMR system to compensate for inhomogeneities in the field of the magnet.
2. Description of the Related Art
A homogeneous magnetic field is critical for NMR experiments. Imaging experiments such as gradient echo-based techniques, localized spectroscopy and echo-planar imaging are a few experiments that can fail if the homogeneity of the magnetic field within a sample being tested is poor. While spin-echo imaging is more tolerant of inhomogeneities, a homogeneous magnet will nonetheless give better results.
In conventional NMR systems, the field of the magnet is "shimmed" by placing appropriate "shim" currents through shim coils located at various positions relative to the field of the magnet. As appropriate currents are passed through the shim field coils, they produce characteristic fields which compensate for the inhomogeneities of the magnet. However, achieving the appropriate characteristic fields in the shim coils often requires adjusting the current through the coils manually. This procedure can be tedious and time-consuming. In high-resolution systems, it is not unusual to shim a magnet for hours, or even days, to achieve a certain homogeneity specification. This procedure can be automated by employing a simplex procedure, but that too can be time consuming, and may not always give the optimum results.
A more analytical solution to the problem is to determine the inhomogeneous field distribution of the magnet, and calculate the currents of the shim coils necessary to compensate for the inhomogeneities. The field of the magnet can be measured by using a small NMR probe which is moved through the magnetic field by a special mechanical device. However, this method is also time-consuming. Furthermore, the probe itself causes a distortion in the field, and the measurement does not correspond to the actual field that would influence a sample being tested.
A number of automated shimming procedures have been reported which make use of NMR imaging. These methods are beneficial because: 1) the field measurements are done while a sample of interest is present in the field of the magnet, and the field inhomogeneity seen by the sample is directly measured and compensated for; 2) knowing the field or frequency distribution within the sample allows the specification of an arbitrary shimming region of interest; 3) shim field maps are calibrated using the same measurement methods and, therefore, and misregistration in the image caused by misalignments, non-linearities and imperfections in the gradient or shim coils are self-compensated; and 4) the shimming procedures can be fully automated.
There are two basic approaches to measuring frequency or field distribution within a sample of interest: direct frequency measurement methods using spectroscopic imaging techniques; and image based methods in which frequencies are evaluated from the complex image data that is detected.
Frequency measurements based on localized spectroscopy have been described which are analogous to a mechanical point-by-point frequency measurement method. These methods are difficult to perform because they are prone to artifacts caused by eddy currents and signal contribution from outside the selected region of interest. If multiple resonances are present, the voxel or slice selection can be complicated by partial volume or chemical shift effects.
A frequency mapping method based on Chemical Shift Imaging (CSI) uses an RF pulse followed by a set of three nested phase-encoded gradient pulses, after which a signal is acquired. A four-dimensional (4D) Fourier transform is then used to directly obtain the frequency distribution within the sample. A primary disadvantage with this method is that data acquisition times are particularly long due to the use of three phase encoded gradients. Typically, the frequency measurement within each voxel is obtained by analyzing the lineshape. However, this analysis can be more complicated if overlapping resonances are present, in which case post processing of the data is often needed because of complicated line shapes and phase anomalies.
Of the image-based techniques, the gradient echo method has the advantage that the pulse flip-angle can be made short so that the sequence can be repeated with a short recycle time, thereby improving the efficiency of the method. Echo-planar imaging (EPI) is an image-based technique which can reduce the data acquisition time by a factor of about N, where N is the size of one of the phase encoded dimensions. However, the EPI method is extremely sensitive to eddy currents caused by the pulsing of large gradients and also to field inhomogeneities, and special gradient hardware is needed.
It has been shown that the analysis of field maps for shimming can be done by collecting a few one-dimensional (1D) projections instead of an entire three-dimensional (3D) dataset. This method has been demonstrated with a gradient echo imaging method and with a stimulated echo method. The method results in a significant saving in data acquisition times, making it useful for clinical and other applications where fast shimming methods are required. However, these echo methods rely on the frequency shift between images acquired at two different echo times which is, presumably, caused by inhomogeneities in the magnetic field. If the inhomogeneities are too great, a phase wrap occurs between the two echo images, and aliasing results, making frequency measurement impossible. Large inhomogeneities can also lead to signal loss due to dephasing of the spins and severe image distortions.
The subtraction of two phase images to acquire a frequency distribution (as described above) gives a result which is a modulo 2.pi. radian value. Thus, any phase change greater than 2.pi. radians is wrapped (i.e., aliased) in the phase image, and results in discontinuities at the 2.pi. boundaries. If the phase wrapping goes uncorrected, the shim calculations can fail due to errors in the frequency measurement. Previous approaches to the phase wrapping problem include avoiding the problem by keeping the delay between the echo times short enough that the spins do not accumulate a phase change greater than 2.pi.. However, this approach may not be reliable when dealing with high-field systems. The use of higher order shims and large susceptibility effects at these fields usually cause large field gradients in the sample, resulting in phase wraps.